A method for the determination of nucleotide sequences of regions of DNA that are located some distance from restriction endonuclease sites will be tested. An EcoRI restriction fragment of phage gamma DNA will be esterified with sorbitol at one of its 5'-terminal phosphate groups and then subjected to restricted DNase I hydrolysis that is caused by reversible modification of the DNA with a water-soluble carbodiimide. The resulting series of fragments that still contain the sorbitol moiety will be isolated by chromatography on a cellulose derivative that possesses immobilized dihydroxyboryl groups. The members of the series will then be labelled at their 3' terminals with 32P and separated by gel electrophoresis. Sequence information will be extracted by analyzing the nucleotide sequences near the labelled 3'-terminals of these fragments. In addition, the resistance of DNA regions that are modified with a water-soluble carbodiimide to exonucleolytic digestion will be exploited in a method for the isolation of DNA sequences that correspond to deletions or insertions in phage DNAs.